This invention relates generally to transport refrigeration systems and, more particularly, to the cooling of an engine driven electric generator therefor.
Generally, transport refrigeration systems such as those used on truck trailers, have employed belt driven and/or mechanically linked, shaft driven compressor units and/or belt driven, or otherwise mechanically linked, fan powering systems. That is, the unit engine, such as a diesel engine, is mechanically connected to drive both the compressor unit(s) and the unit refrigeration systems fans and other components.
A significant improvement to such an arrangement was a so called “all electric” transport refrigeration system as described in U.S. Pat. No. 6,223,546, assigned to the assignee of the present invention. In such a unit, a generator is mechanically connected to the system engine and is used to generate the electrical power needed to drive the compressor and the various refrigeration system fans and other components.
Heretofore, such an engine driven generator has been cooled by a single stage centrifugal fan disposed on one end thereof and adapted to circulate air over the stator and armature components for the cooling thereof.
Relatively low engine speeds are desired in trailer refrigeration systems in order to attain low fuel burn rates. This limitation on shaft speed results in relatively low pressure rise capability from a single stage fan. Yet, high pressure rise is needed to drive flow through the generator windings and rotor-stator air gap. System space constraints make large cooling fans or push-pull fan options impractical.
Augmentation of a cooling fan by the use of a second fan in cooling rotating machinery as shown in U.S. Pat. Nos. 6,700,235 and 7,129,604. However, the fans are mounted at opposite ends of the generator such that one of the fans is blowing cooler air across the rotating machinery, while the other fan is drawing warmer air away from the rotating machinery. This is disadvantageous since the warmer air is less dense and therefore results in reduced air mass flow rate.